Thermal spraying involves the heat-softening of the heat-fusible material, such as a metal or ceramic and the propelling of the softened material in particulate form against a surface to be coated to which the heat-fusible material bonds. A thermal spray gun is usually used for this purpose and with one type, the heat-fusible material is supplied in powder form to the gun. The powder is of quite small particle size, e.g. below about 150 microns (100 mesh U.S. Standard screen size) and as small as one micron, and is difficult to meter and control. Typically a thermal spray powder is -100 +325 mesh (-149 +44 microns) or -44 microns +15 microns.
A thermal spray gun normally utilizes a combustion flame or a plasma flame to effect melting of the powder, but other heating means, such as electric arcs, resistance heaters or induction heaters can also be used, alone or in combination. In a powder-type combustion flame spray gun, the carrier gas for the powder can be one of the combustion gases or compressed air. In a plasma flame spray gun the carrier gas is generally the same as the primary plasma gas, although other gases such as hydrocarbon are used in special cases.
To obtain high quality coatings, it is necessary to accurately control the rate of the powder fed through the gun and to maintain the same constant for a given set of spray conditions. The type of fine powder used is a very difficult material to handle and to feed with any uniformity into a carrier gas.
Various feeding systems have been devised which involve pressure regulation, fluidization and feedback principles in the control of powder feed rate, replacing mechanical conveyance which subjects components to severe wear and attendant loss of rate control. For example, U.S. Pat. No. 3,501,097 (Daley) discloses a mechanical feed device. In FIG. 7 thereof such a device is shown that utilizes differential pressure between the feeder and the carrier gas conduit to regulate the drive motor for the mechanical feeder. In FIG. 8 thereof a pressure regulator injecting carrier gas into a powder chamber replaces mechanical feed and preset pressure regulates feed rate.
Feeders with powder fluidization are typified by U.S. Pat. No. 3,976,332 (Fabel) which discloses a feeder utilizing a fluidic amplifier, and U.S. Pat. No. 4,561,808 (Spaulding et al) which involves a simple pressure regulator. In each of these patents, control of a feed gas separately from the carrier gas is used to control feed rate. Under the operating principle of these patents, for example with a pressure regulator, feed gas pressure is set at a constant level slightly above atmospheric pressure independent of carrier gas pressure. Pressure to the feeder from the powder-carrier gas conduit is proportional to powder feed rate in the carrier conduit. A rise in feed rate decreases the differential between the feed gas pressure and the carrier gas pressure, resulting in a compensating decrease in feed rate. These references thus teach regulation of feed rate via pressure feedback.
A similar concept is disclosed in U.S. Pat. No. 2,623,793 (W. H. Hill) in which pressure differential is detected between the input and output portions of a lift pipe for conveying subdivided solids. The pressure differential is translated to operation of a mechanical valve for a feed gas. U.S. Pat. No. 3,599,832 (Smith) teaches such pressure differential to mechanically control a valve in the powder-carrier conduit.
U.S. Pat. No. 3,365,242 (Metz) similarly involves several flows including a carrier gas and a fluidizing gas. The latter is returned through a bleeder line from the top of the hopper to the powder-carrier line. More generally, feed gas may be introduced above the powder (as in Fabel), below the powder (as in Spaulding), or through a tube from the top as disclosed in German Pat. No. G8417749.7.
U.S. Pat. No. 4,747,731 (Nagasaka et al) is more complex, with differential pressure taps in the carrier passage operating a control valve for adding gas downstream to the carrier gas, to increase degree of vacuum for drawing in powder from an open hopper. A gas operated pinch valve is separately operated to turn powder feed on and off.
Variations of systems for bypassing carrier gas around the feeder are known. U.S. Pat. No. 3,291,536 (Smoot) discloses a bypass and several pinch valves operated from pressure switches sensitive to input pressure of the carrier gas, to bypass the feeder and blow out slugs from the carrier line when backpressure develops from powder slugs in the line. U.S. Pat. No. 3,432,208 (J. A. F. Hill et al) shows a bypass line for adding carrier gas in a system with manually operated valves, for feeding into a high pressure wind tunnel. U.S. Pat. No. 4,740,112 (Muehlberger et al) discloses bypassing of the feeder by the carrier gas when a mechanically driven feeder is turned off. By reference therein the Muehlberger system is directed to feeding into low pressure plasma spraying chambers.
The aforementioned references are at least implicitly directed to feeding powder in a carrier gas to a utilization device such as a conventional thermal spray gun that produces little or no significant changes in backpressure, except Smoot which addresses pressure surges due to slugs in the line (and bypasses the feeder to blow out the slugs). The feeders of Fabel and Spaulding may stop feeding completely if there is a high backpressure, since the differential pressure of the feed gas will go to zero (or even reverse). Non-mechanical, fluidized powder types of feeders have otherwise been quite successful for feeding powder to thermal spray guns having relatively low backpressure (relative to atmospheric), including a plasma spray gun having external powder injection.
However, internal injection plasma guns with significant backpressure generally continue to require feeders with mechanical metering. A further problem has evolved with the commercial development of high velocity combustion spray thermal spray guns such as a Metco Type DJ Gun produced by The Perkin-Elmer Corporation and disclosed in copending U.S. patent application Ser. No. 193,030 filed May 11, 1988 (Rotolico et al) [Attorney Docket No. ME-3818] assigned to the present assignee. These guns develop substantial changes in backpressure from essentially atmospheric to 50 to 60 psig or more and, furthermore, the backpressure depends on exact gun conditions and is not predictable for presetting the powder feeder gas pressures. Also, since the backpressure is nearly zero before lightup of the gun, with prior art feeders, there can be time delays for the feeder pressures and feed rates to adapt when the gun is lit.